Digital watermarking is a burgeoning science with an extensive literature. Some of it, by the present assignee, relates to texturing the surface of a paper to convey optically-detectable digital information.
The assignee's U.S. Pat. No. 5,850,481, for example, details how the microtopology of a paper can be shaped, e.g., by a Braille-like machine, to encode digital data. The assignee's application Ser. No. 09/127,502 (now U.S. Pat. No. 6,345,104) details how the high-pressures used in intaglio printing can be used to similar effect.
In accordance with a preferred embodiment of the invention, paper is textured during the paper-making process, rather than in a subsequent process, thereby affording various advantages. The texture yields a known signature when transformed to the frequency domain. This signature can be used for various purposes, including determining the angular orientation of the paper when scanned and analyzed for the presence of watermark data, and for simple document recognition purposes (e.g., photocopiers that refuse to reproduce banknotes). One way of effecting the desired texturing is by shaping the surface of a de-watering element accordingly.
The foregoing and additional features of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.